The "Tree of Life" is an ancient philosophy common to many cultures and mythologies. Often regarded as an all-nourishing, all giving Mother, many myths speak of the Tree of Life or World Tree as involved in the creation of the universe. Britain was once covered by mightly oak forests, and tree reverence is a major feature within Celtic religion. Trees reflect a link between the upper and lower worlds. The Ancients envisioned the entire cosmos in the form of a tree whose roots grow deep into the underground, its branches reaching high into the heavens and sky. Druids had their teaching centers in the midst of Oak groves, and the words for wood and wisdom are similar (Welsh gwydd and gwyddon).